Magnetic recording stores information in a hard magnetic layer of a magnetic medium by setting the direction of magnetization of regions of the layer. A recording apparatus of the type using a magnetic disk, magnetic tape or similar magnetic recording medium, reproduces stored digital data in the form of an analog waveform representative of transitions of magnetic polarity on the recording medium. In the case of data recorded in an NRZ (non-return to zero) or NRZI (non-return to zero inverted) recording pattern, for example, positive and negative peaks follow one another in succession with transitions between the peaks passing through the signal baseline. Depending on the code useca, the positive and negative peaks may represent explicit data bit values, in self-clocking codes, or may represent explicit clock bits and encoded data bits in pulse position or interval modulation codes. One form of a digital data reproducing circuit for reading any such recorded code differentiates the reproduced waveform and determines the points where the differentiated waveform crosses a zero level representative of zero AC volts, to detect the peaks.
In commonly assigned U.S. Pat. No. 4,977,419, to Wash et al., a photographic filmstrip having a virtually transparent, magnetic film layer covering the non-emulsion side of the filmstrip (referred to as an MOF layer) is disclosed in conjunction with various camera systems. Potential longitudinal recording tracks are illustrated in the MOF layer in both the image frame area and on either side of the image frame area for recording information such as film type, film speed, film exposure information and information relevant to the processing and subsequent use (e.g., printing) of the exposed image frames. The systems disclosed therein provide for recording of information during film manufacture, reading and/or recording of information on certain tracks during camera use, and reading and/or recording of printer related information during photofinishing.
Novel methods for modulating the binary data or information into a format suitable for recording and reproducing in such tracks in the camera shown in the '419 patent are disclosed in commonly assigned U.S. Pat. No. 4,964,139, also to Wash et al. Two self synchronizing PPM coding methods are disclosed in the '139 patent, as the "Wash encoded signal" and the "Chi encoded signal" methods. Both PPM encoding methods have similarities as pointed out in the '139 patent, and the Wash encoded signal method is further referred to herein for purposes of explaining how the present invention may be practiced in at least one of its preferred embodiments in locating drop-outs in magnetically recorded data encoded in such a manner.
An example of the Wash encoding method, as used to record data in bit streams in the MOF layer tracks described in the '419 patent, is shown in FIG. 1 which essentially reproduces FIG. 3 of the '139 patent. In FIG. 1A), two successive bits 0 and 1 are position encoded in the two corresponding "interval or information-cells" as positive going data signal level transitions bounded by negative going clock signal transitions between the directions of magnetization. The Wash self-clocking bit code is effected by the position of the positive going data signal level transition in the information-cell. If it is positioned within the first half of the duration of the information-cell, then a data bit 0 is recorded. Similarly, a data bit 1 is recorded by positioning the data transition in the second half of the information-cell. As an aside, it should be noted that the data bit may in fact be a parity bit, as the two terms are used below.
Note that the encoding of the information or data transitions for both the first and second information-cells leaves invariant the clock transitions. With this self-clocking code, film transport velocity can vary during recording and playback without affecting the ability to synchronize and read the recorded data. As a result, the information-cells may lengthen or shorten depending on the filmstrip velocity, but the relative position of the data transitions can still be decoded. Thus, the camera disclosed in the '419 patent, for example, may record data in the MOF layer tracks while winding the filmstrip between exposures without imposing any velocity controls or recording an independent clock track. In fact, large amounts of jitter in the camera transport at frequencies at or near the data frequencies precludes the use of a phase-locked loop in generating a clock during magnetic signal read out.
The PPM information-cells are decoded by means of a head sensitive to the field or rate of change of the flux emanating from the polarized regions at the transition zones. The signal produced by most magnetic reproduce heads is a voltage pulse corresponding to the location of a transition zone from one magnetically set region to another. The detection circuitry to which the head is attached must determine the location of one peak relative to another. This is most often done by differentiating the pulse and comparing the voltage to zero. The differentiated waveform will cross zero at a time corresponding to the peak in the original waveform, at which point the comparator will change state. This works well in the region where the pulse has energy. However, outside this region, the input: to the comparator hovers around zero, crossing zero as a result of noise, producing transitions at the output. To remedy this, the output of the comparator is gated by a circuit which only allows the comparator state to change when the input waveform exceeds a threshold, usually 30%-45% of the peak amplitude. This scheme works well in eliminating the unwanted transitions. A side effect is that, if the peak amplitude of the input signal fails to reach the threshold, no transition will occur and the data will be missed. Hard disk drives deal with this problem by marking locations on the disk where this occurs and not using the areas. Tape drives use frequent re-synching, read-while-write, re-reading, and powerful error correction code (ECC) to deal with the problem. Simple, inexpensive systems for use in cameras cannot use such complex and expensive solutions, and resort is therefor made to the self clocking PPM coding of Wash or Chi.
As shown in FIG. 1B, the data and clock transitions are read out as positive and negative going spike signals at the transitions. Turning to the read circuit 10 of FIG. 2, reproduced from the '139 patent, a read/write magnetic head 12 provides the read signals of FIG. 1B on lines 14. A pre-amplifier 16 amplifies the output signal from the magnetic head 12 and applies the amplified signal on lines 26 to a filter 18 for removing an unnecessary component from the amplified read signals. A post-amplifier 20 amplifies the filtered read signals and applies the filtered and amplified, positive and negative, signals to a detector 24 via line 22. In detector 24, the signals are applied to a positive peak detector (NPD) 30 and a negative peak detector (PPD) 32 for detecting the negative and positive pulses having amplitudes exceeding respective threshold levels to ensure that noise signals are not decoded as data or clock signals. Uniform amplitude, polarity, and pulse width, clock and data, signal pulse trains are thereby generated on lines 38 and 34 as depicted in FIG. 1C. A delay circuit 40 is connected to the NPD 32 via line 38, resulting in a delayed clock pulse train on lines 46 and 48 and also appearing in FIG. 1C.
The delayed clock signal is applied to the set terminal of the flip-flop 36 and is also applied to clear the up/down counter 50. The data signal is applied to the clear terminal of the flip-flop 36, so that the flip-flop 36 is set by delayed clock signals and cleared by data signals to provide the square wave signal shown in FIG. 1D at the Q output. The Q output is set high by a delayed clock signal and low by the succeeding data signal.
During the high state, the up/down counter 50 counts system clock 56 pulses applied to its CLK input, incrementing the count when the Q out, put is high and decrementing the count when the Q output is low. The most significant bit (MSB) of the output count of the up/down counter 50 and the CLOCK output from the NPD 32 are supplied to a computer 42. The particular manner of determining the data bit in the information-cell is further explained in the '139 patent.
Because of the pulse position modulation, variation in velocity of the filmstrip during recording or readout (within reasonable bounds in which a camera would be specified to operate) does not effect the accuracy of the data, as described above. However, the compliance of the magnetic record/reproduce head with the low density MOF layer during recording or reproducing the recorded data itself may be faulty for other reasons leading to a failure to either read or write data in one or more of the information-cells.
When compliance with the magnetic head is lost due to a dirt speck on the magnetic medium or curl of the medium or jitter in the head suspension or if the magnetic recording medium has a defect such as a change in the density of the magnetic powder or if electrical noise is introduced in a playback system from outside the system, false data generally referred to as a "drop-in" and/or the omission of data generally referred to as a "drop-out" can occur in the reproduced data stream. As described above with respect to FIG. 2, the conventional data read out circuit compares the absolute positive and negative amplitudes of the reproduced analog waveforms to threshold levels in the PPD and NPD circuits to convert, among the above-mentioned peaks, only the peaks having amplitudes higher than a predetermined threshold level into digital signals, determining that they are data bits 1 or 0 depending on their position in the case of the PPM encoded signals. Thus, if the amplitudes of the pulse signals fade below the thresholds, both the clock pulses separating successive information-cells and the data pulses signifying the binary data content can be lost.
The information-cells are recorded in a pattern of multi-bit bytes described in detail in the '419 patent, particularly in regard to FIGS. 6-9 thereof. A simplification of that format appears as follows in Table I:
______________________________________ Original Data ______________________________________ P.sub.1 d.sub.11 d.sub.12 d.sub.13 d.sub.14 d.sub.15 d.sub.16 d.sub.17 P.sub.2 d.sub.21 d.sub.22 d.sub.23 d.sub.24 d.sub.25 d.sub.26 d.sub.27 P.sub.3 d.sub.31 d.sub.32 d.sub.33 d.sub.34 d.sub.35 d.sub.36 d.sub.37 P P P P P P P P ______________________________________
A block of three data bytes of eight information-cells is shown. The first bit of each byte is a parity bit (P.sub.i), and the remaining seven bits are data bits (d.sub.i1 -d.sub.i7). Each block of bytes recorded serially in a single track in the MOF layer is separated by a parity byte, called a Longitudinal Redundancy Check (LRC). Each vertical column bit value of the LRC is calculated from the values of the data bits in its respective column. The data bits of successive information-cells in each block are thereby protected with a 2-dimensional, horizontal and vertical, parity check in the data blocks. This scheme is able to correct an odd number of bits in error in any one byte, provided a bit is present at every position and the LRC is present in its proper position with the longitudinal parity bits lined up with their respective columns.
The PPM encoding method described above incorporates the clock information in with the data so that a drop-out typically results in both loss of data content and loss of the clock signal. When a drop-out occurs, no clock is available to transmit data from the decoder to the memory, so that bits are completely missing in the recovered data block. This results in loss of byte synchronization and renders the parity checks useless in recovering the uncorrupted data. Shown below in Table II is the situation after recovery when bits are lost:
______________________________________ Corrupted Data ______________________________________ P.sub.1 d.sub.11 d.sub.12 d.sub.13 d.sub.14 d.sub.15 d.sub.16 d.sub.17 P.sub.2 d.sub.21 d.sub.22 d.sub.23 d.sub.27 P.sub.3 d.sub.31 d.sub.32 d.sub.33 d.sub.34 d.sub.35 d.sub.36 d.sub.37 P P P P P P P P X X X ______________________________________
A drop-out of three information-cells and the respective data bits d.sub.24, d.sub.25, d.sub.26, occurred in the second row, causing the data bytes and the LRC parity bits to shift in the recovered block. No correction is possible because of this, and the data read out may either be thwarted or the data may be misinterpreted. It should be understood that in practice, the problem is magnified by the number of bytes in the data set and the location of the drop-out in the series of bytes.